1) Your question and scale are a sexist representation of relationships, being that the man should be older than the woman in order to be acceptable. The question should be rephrased to older people dating younger people and your calculation should be changed to 1.4Y=Omax (y=younger person, o=older person)
2) It's a cute scale that's been done about a thousand times before
3) It has to do with social, intellectual and class markers. For example, 16-18 is an important marker as 16 is considered the "right" time to start dating and 18 is the first cutoff of legality (if you're >18, it can be illegal to be with a younger than 18 person). This also coincides fairly well with college. The next marker is 21 (you know, drinking in the US) or 22 (graduation from college). the 18-22 range is generally one associated with youthful indiscretion, staying up late and stupid mistakes. After that, 25 is a good age to signify growing maturity (no more silly tshirts, no more glow in the dark posters on the wall) while still showing youth. After that, the decades mark growing 'maturity' 30 being a youngish professional (emphasis on professional) 40 middle age, 50 older individual, etc., etc. The key is that when one dates much outside of these little scales, it causes social awkwardness. you can't bring an under 21 to bars, those in college can't relate much to a 9-5 workweek, those barely out of college don't have the appreciation of stability/routine that those that have been working for a decade have, etc. While people certainly can bridge such gaps, there are more chances for awkwardness, misunderstandings and conflicting goals, as well as raising questions of motive. What does a 21 yo want with a 40yo (answer: generally money related) and what does a 40yo want with a 21yo (generally sex/youth recapturing related).
So it really breaks down to a questioning of the status quo freaks people out, so as long as you float in the same circles, you're good to go. (Which means that a 24 year old that skipped college to work and is in middle management is able to float in the same circles that a 32yo who went to college and is in a similar position. There is certainly an age discrepancy, but interests/positions will be more aligned).
~Jason